The present invention relates to a magnetic resonance imaging apparatus, a magnetic resonance imaging method, a diffusion tensor color map image generating apparatus, and a diffusion tensor color map image generating method.
The magnetic resonance imaging (MRI) apparatus is in wide use particularly in medical applications as an apparatus for radiographing tomographic images of a subject with use of a nuclear magnetic resonance (NMR) phenomenon.
In the magnetic resonance imaging apparatus, a radiographing area of a subject is accommodated within a static magnetic field space, thereby aligning proton spins in the radiographing area in the direction of the static magnetic field to generate a magnetization vector. Then, RF pulses of a resonance frequency are emitted to create a nuclear magnetic resonance phenomenon, thereby causing the spins to flip and changing the magnetization vector of the protons. Thereafter, the magnetic resonance imaging apparatus receives a magnetic resonance (MR) signal which is produced when the protons revert to the state of the original magnetization vector, and reconstructs an image such as a slice image related to the radiographing area on the basis of the received magnetic resonance signal.
In the magnetic resonance imaging apparatus, for example in order to obtain information such as one related to a running direction of a fiber such as a nerve fiber, a fiber tracking method is performed in the diffusion tensor method to track the fiber. More particularly, the fiber tracking is conducted by calculating a primary axis direction of a diffusion tensor. Further, in order to grasp the running direction of the fiber, a diffusion tensor color map image is reconstructed with respect to a curved surface along which a tract extracted in the fiber runs, and the reconstructed diffusion tensor color map image is displayed (see, for example, Patent Literatures 1, 2, 3 and Non-Patent Literature 1).
Here there is utilized a phenomenon such that when a fiber such as a nerve fiber and water molecules present around the fiber are diffused in a direction parallel to a running direction of the fiber, the diffusion is not limited, but when the diffusion occurs in a direction different from the fiber running direction, the diffusion is limited by a cell membrane, so that a measured diffusion coefficient of the water molecules becomes high in the portion parallel to the fiber running direction, while it becomes low in the portion not parallel to the running direction. That is, imaging is made as a diffusion tensor color map image by utilizing the anisotropy of the water molecule diffusion.
[Patent Literature 1]. Japanese Unexamined Patent Publication No. 2004-89224.
[Patent Literature 2]. Japanese Unexamined Patent Publication No. 2004-81657.
[Patent Literature 3]. U.S. Pat. No. 6,526,305.
[Non-Patent Literature 1]. Shigeki AOKI, et al., “Mechanism of Diffusion MRI,” Shujun-sha.
For generating a diffusion tensor color map image, as described above, scanning is performed in a static magnetic field space for a three-dimensional area including the brain of a subject for example as a radiographing area, thereby acquiring a magnetic resonance signal from the radiographing area.
For example, scanning is performed in the radiographing area which is a three-dimensional area with respect to both the case where MPG (Motion Probing Gradient) is applied in plural directions and the case where MPG is not applied.
Next, a fiber such as a nerve fiber running in the radiographing area is tracked on the basis of a diffusion tensor calculated from the magnetic resonance signal.
After the diffusion tensor is calculated as a symmetric matrix of 3 rows by 3 columns from the magnetic resonance signal obtained in the above execution of scanning, there are calculated a maximum eigen value of the diffusion tensor and a principal axis vector as an eigen vector corresponding to the maximum eigen value. Thereafter, on the basis of both the maximum eigen value and the principal axis vector, pixels corresponding to the fiber such as a nerve fiber running in the radiographing area are tracked by such a fiber tracking method as described above.
Next, in the tracked fiber, a diffusion tensor color map image is reconstructed so as to correspond to a curved surface including a tract which is extracted for example as a cone path.
FIG. 5 shows an outline of the reconstructed diffusion tensor color map image. In FIG. 5(a), fibers tracked in a radiographing space of a subject are shown schematically in terms of xyz coordinates. On the other hand, 5(b) shows an outline of the generated diffusion tensor color map image, in which, in the diffusion tensor color map image, the portion indicated in “red” color is hatched with left oblique lines as leftward inclined perpendiculars, the portion indicated in “green” color is hatched with right oblique lines as rightward inclined perpendiculars, and the portion indicated in “blue” color is a pattern-free portion.
As shown in FIGS. 5(a) and 5(b), three primary colors are allocated to pixels in according with fiber running directions so as to provide indication by “blue” when the fiber running direction is y direction, provide indication by “red” when the fiber running direction is z direction, and provide indication by “green” when the fiber running direction is x direction, in a curved surface CS in which the diffusion tensor color map image is generated. The diffusion tensor color map image is reconstructed by thus making the pixels different in color.
That is, in generating the diffusion tensor color map image, as shown in the following expression A, components e1x, e1y and e1z in an xyz coordinate system of a magnet are determined from a principal axis vector e1 and three primary colors, i.e., RGB, are allocated to those components e1x, e1y and e1z:(R,G,B)=(e1x,e1y,e1z)  (A)
Therefore, in the case where a tract meanders or runs while bending as in a region R11 indicated by a dotted line in FIG. 5(b), the pixels corresponding to a fiber F1 running as a tract include a portion displayed by plural different colors. When this diffusion tensor color map image is observed, there sometimes is a case where it is difficult to grasp the running direction of the tract exactly. Such an inconvenience may occur also when the head of a subject is inclined relative to the coordinate axis of the xyz coordinate system at the time of execution of scanning, thus making it difficult to improve the diagnostic efficiency.